Fototinrûlijhaedje bedot orayes dizeutrin

Photosensitivity Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicity. ...

is an abnormal skin reaction to direct sunlight exposure. It is unrelated to a

sunburn Sunburn is a form of radiation burn that affects living tissue, such as skin, that results from an overexposure to ultraviolet (UV) radiation, usually from the Sun. Common symptoms in humans and animals include: red or reddish skin that is ho ...

. These reactions are due to photosensitization, the accumulation of photosensitive compounds beneath the skin. In some cases, the photodynamic substances come from ingested plants or drugs, after being metabolized or not. In other cases, the photodynamic substances may be produced in the body itself due to inborn errors in pigment metabolism, especially those involving the

heme Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver. In biochemical terms, heme is a coordination complex "consisti ...

synthesis. Photosensitivity reactions are usually seen in herbivorous or omnivorous animals, though such reactions are known among carnivores.

Effects of photosensitivity

Photosensitivity reactions are characterized by severe

inflammation Inflammation (from la, wikt:en:inflammatio#Latin, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or Irritation, irritants, and is a protective response involving im ...

of the skin with depigmentation and ulceration. In some cases (Type I), pigments are excreted through urine or deposited in hard tissues such as bones and teeth, causing discolouration. Affected animals should be protected from direct sunlight until the photodynamic compounds have been completely eliminated from the body or the appropriate treatments have been given.

Biological mechanism of photosensitivity reactions

Photodynamic compounds are activated by sunlight, which excites them to a higher energy state. They immediately release the excess energy and transfer it to the surrounding acceptor molecules, such as enzymes or free radicals. These molecules are thus activated and cause the skin reaction.

Classification of photosensitivity reactions

Photosensitivity reactions are classified by origin into four types: Type I, Type II, Type III and Type IV.

Type I Photosensitivity

Type I Photosensitivity occurs when the photodynamic compounds need not undergo metabolism to be activated and so may accumulate under the skin directly following ingestion. Examples of plants producing Type I Photosensitivity include

biserrula ''Biserrula'' is a genus of flowering plants in the family Fabaceae, found in Portugal (including Madeira and Selvagens), the Canary Islands, all the Mediterranean countries, the Arabian peninsula, and in east Africa as far south as Tanzania. '' ...

(''Biserrula pelecinus''),

Buckwheat Buckwheat (''Fagopyrum esculentum''), or common buckwheat, is a flowering plant in the knotweed family Polygonaceae cultivated for its grain-like seeds and as a cover crop. The name "buckwheat" is used for several other species, such as '' Fago ...

and

St John's wort ''Hypericum perforatum'', known as St. John's wort, is a flowering plant in the family Hypericaceae and the type species of the genus ''Hypericum''. Possibly a hybrid between '' H. maculatum'' and '' H. attenuatum'', the species can be found a ...

(''Hypericum perforatum''). Certain drugs such as

phenothiazine Phenothiazine, abbreviated PTZ, is an organic compound that has the formula S(C6H4)2NH and is related to the thiazine-class of heterocyclic compounds. Derivatives of phenothiazine are highly bioactive and have widespread use and rich history. T ...

or its derivatives may also lead to photosensitivity reactions, commonly in sheep. Phenothiazine is converted by the body into the photodynamic compound phenothiazine sulfoxide.

Type II Photosensitivity

Type II Photosensitivity is caused by inborn errors in the metabolism of certain biological pigments. In the absence of some key metabolic enzymes, the products of intermediary metabolism accumulate. They are either eliminated through the urine and body fluids or are deposited in some body tissue, such as bone and teeth. A common condition seen in animals is

congenital porphyria A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can r ...

due to the accumulation of Uroporphyrin, which is deposited in the teeth and bones, giving them a pink discolouration, or excreted through the urine, exhibiting a pinkish

fluorescence Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

under ultraviolet light.

Type III Photosensitivity

Type III (aka hepatic photosensitivity) is the most common type of photosensitivity reaction seen in animals. In this type, the photodynamic substance is phylloerythrin, a derivative of chlorophyll produced in the body. Normally, phylloerythrin is secreted into the intestine by the biliary system and excreted through the feces. In Type III Photosensitivity, damage to the

biliary transport mechanism A bile duct is any of a number of long tube-like structures that carry bile, and is present in most vertebrates. Bile is required for the digestion of food and is secreted by the liver into passages that carry bile toward the hepatic duct. It ...

prevents the excretion of phylloerythrin, allowing the photodynamic substance to enter the circulatory system and accumulate under the skin. This condition is exacerbated by the presence of hepatic parasites.

Type IV Photosensitivity

Type IV Photosensitivity occurs following the ingestion of certain plants, such as

alfalfa Alfalfa () (''Medicago sativa''), also called lucerne, is a perennial flowering plant in the legume family Fabaceae. It is cultivated as an important forage crop in many countries around the world. It is used for grazing, hay, and silage, as w ...

. The reason is obscure or idiopathic.

Treatment

Affected animals should be transferred to shade. To overcome shock,

corticosteroid Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex of vertebrates, as well as the synthetic analogues of these hormones. Two main classes of corticosteroids, glucocorticoids and mineralocorticoids, are involv ...

therapy and fluid infusions may be tried.

Antihistamines Antihistamines are drugs which treat allergic rhinitis, common cold, influenza, and other allergies. Typically, people take antihistamines as an inexpensive, generic (not patented) drug that can be bought without a prescription and provides re ...

are also useful.

References

{{Commons category, Photosensitivity in animals Animal diseases ru:Фотосенсибилизация